There is currently an ongoing engineering effort, by suppliers of railway braking equipment, to develop an electro-pneumatic type brake system for railroad freight trains. It is generally acknowledged in the railroad industry that the development of such an electro-pneumatic brake control will substantially enhance the operation of a train by achieving a faster brake response, more equalized car retardation and a generally more uniform braking effort throughout a long train of cars.
These enhanced results are based on the assumption that all of the cars, or at least a majority of the cars, making up a train will be appropriately equipped for utilizing such improved electro-pneumatic braking system, in which case direct control of the brake cylinder air pressure is envisioned. With the possible exception of certain unit trains, however, it currently cannot be reasonably expected that any such majority of cars will be immediately implemented with the required electro-pneumatic brake equipment.
Accordingly, for the present, indirect brake cylinder air pressure control is contemplated, in which the train brake pipe air pressure is controlled not only at the locomotive, but also at one or several remote cars located throughout the train to accelerate the reduction of brake pipe air pressure in order to achieve a faster and more uniform brake response.
When the brake pipe pressure is reduced at any given point along the brake pipe, however, a transient low pressure is created in the vicinity of that point. Consequently, if the exhaust creating the transient low pressure is closed too quickly, i.e., before the entire length of the brake pipe pressure is stabilized, air will flow towards the low pressure point thereby causing a localized increase in pressure at that point which can cause control valves in that vicinity to inadvertently release.
On the other hand, if the exhaust is closed too slowly, the pressure will be reduced to a level below the desired target pressure. Therefore, anytime the air pressure is reduced at any one or more remote locations along the brake pipe, it is vitally necessary that a flow equilibrium be established and maintained thereat following reduction of the brake pipe pressure to a commanded target value. This then requires that an exhaust flow be carefully controlled to continuously match the changing flow of air from the rest of the brake pipe to the exhaust location.
At the present time, most railway freight trains are required to utilize an end-of-train unit disposed on the last car in the train. Such an end-of-train unit, among other highly critical functions, may be equipped to independently and remotely initiate a reduction of the brake pipe air pressure from the rear of the train in response to the train operator's activation of a special triggering device located in the cab of the locomotive. This may be accomplished, for example, by the operator transmitting a brake application command signal from the locomotive to the end-of-train unit via radio communication.
One approach to effecting such an air pressure reduction in the brake pipe has been to utilize a control valve which includes a variable type orifice through which the brake pipe air pressure is discharged. Normally, in these self-regulating type valves, either a spring setting against a pressurized area or a reference control pressure is utilized to effectively control the variable orifice.
In contrast thereto, nonself-regulating type valves require the use of a microprocessor to constantly monitor the brake pipe pressure to obtain pressure feedback information in order to establish an appropriate valve orifice size in response to a changing difference between the commanded target pressure and the feedback pressure. The above referenced co-pending patent application, Ser. No. 08/562,961, teaches one such nonself-regulating valve system utilizing a valve body having a single annular valve seat encircling a conical protrusion through which the brake pipe pressure is exhausted for both service and emergency brake applications.